Improved JavaScript performance for https://codereview.chromium.org//171773003

sdk/lib/_internal/lib/js_string.dart

Index: sdk/lib/_internal/lib/js_string.dart
diff --git a/sdk/lib/_internal/lib/js_string.dart b/sdk/lib/_internal/lib/js_string.dart
index 6482d8bf0dc0c08f8bab289be3b2604cfd66506f..df97c9c63e89b98bf571596f4ff4f0f466048a7d 100644
--- a/sdk/lib/_internal/lib/js_string.dart
+++ b/sdk/lib/_internal/lib/js_string.dart
@@ -226,34 +226,30 @@ class JSString extends Interceptor implements String, JSIndexable {
     return JS('String', r'#.substring(#, #)', result, startIndex, endIndex);
   }
 
-  String repeat(int times, [String separator = ""]) {
-    if (times < 0) throw new RangeError.value(times);
-    if (times == 0) return "";
-    if (separator.isEmpty) {
-      return JS('String', "new Array(# + 1).join(#)", times, this);
-    } else {
-      var list = new JSArray.growable(times);
-      for (int i = 0; i < times; i++) list[i] = this;
-      return JS('String', "#.join(#)", list, separator);
+  String operator*(int times) {
+    if (0 >= times) return '';  // Unnecessary but hoists argument type check.

[Lasse Reichstein Nielsen, 2014/02/20 06:51:34]

Not unnecessary since you use >>> below (and === 0 as guard). A negative number
would look positive after shifting.

+    if (times == 1) return this;
+    var result = '';
+    var s = this;
+    while (true) {
+      if (times & 1 == 1) result = s + result;
+      times = JS('JSUInt31', '# >>> 1', times);

[Lasse Reichstein Nielsen, 2014/02/20 06:45:09]

What if times is greater than 32 bits? (which would be silly, but  "" * 
10000000000000 is still valid).

[Lasse Reichstein Nielsen, 2014/02/20 06:51:34]

In particular "0x100000000" (1p32) would be treated as zero, so  "a"*0x100000000
returns "". We should throw an out-of-memory error in this case, somehow.

+      if (times == 0) break;
+      s += s;
     }
+    return result;
   }
 
-  String padLeft(int newLength, String padding) {
-    if (padding.length != 1) throw new ArgumentError(padding);
+  String padLeft(int newLength, [String padding = ' ']) {
     int delta = newLength - this.length;
     if (delta <= 0) return this;
-    var list = new JSArray.growable(delta + 1);
-    list[delta] = this;
-    return JS("String", "#.join(#)", list, padding);
+    return padding * delta + this;
   }
 
-  String padRight(int newLength, String padding) {
-    if (padding.length != 1) throw new ArgumentError(padding);
+  String padRight(int newLength, [String padding = ' ']) {
     int delta = newLength - this.length;
     if (delta <= 0) return this;
-    var list = new JSArray.growable(delta + 1);
-    list[0] = this;
-    return JS("String", "#.join(#)", list, padding);
+    return this + padding * delta;
   }
 
   List<int> get codeUnits => new _CodeUnits(this);